Abstract
The western interior of North America contains a lineament of rift-related Cambro-Ordovician intrusions. Many of the intrusions are low in silica, making up a diverse suite of syenites, carbonatites, gabbros, and granites. Throughout the Cambro-Ordovician magmatic belt, intrusions have elevated REE, Nb, Th, and Zr contents. Cambro-Ordovician intrusions in Idaho are no exception to this trend and contain some of the greatest enrichments of incompatible elements among magmatic rocks in the state. Mafic rocks within this suite are nearly as REE-rich as adjoining syenites, and may represent metasomatized mantle partial melts. Syenites, though not significantly more enriched in REE's than their mafic counterparts, demonstrate significant spikes in Zr and Nb content.